The present invention relates to a method for contact drying a paper web wherein the paper web is dried by means of heated smooth-faced drying cylinders. The web is carried through several successively arranged so-called normal groups with single-wire draw in which the drying cylinders are situated in an upper row and reversing suction cylinders or equivalent suction rolls are situated in a lower row below the upper row of drying cylinders. In the method, after the press section of the paper machine, the paper web is dried initially in a number of successively arranged groups with single-wire draw by carrying the web on a drying wire in each group which presses the web against the heated faces of the drying cylinders in that group. In each group with single-wire draw, the paper web is passed on support of the same drying wire from one drying cylinder to another over a reversing suction cylinder or roll. When the paper web is situated at the side of the outside curve of the reversing suction cylinder or roll, the web is kept on the drying wire by means of a difference in pressure against the effect of centrifugal forces.
Further, the present invention relates to a dryer section of a paper machine having an initial part including several successively arranged so-called normal groups with single-wire draw in which the drying cylinders are situated in an upper row and reversing suction cylinders or equivalent suction rolls are situated in a lower row below the upper row of drying cylinders. Between the normal groups, the paper web to be dried has closed draws over the group gaps. The reversing suction cylinders or equivalent suction rolls are arranged so that at least the turning sectors for the drying wire over the cylinders or rolls are subjected to a vacuum.
In web formation and processing applications, the highest web speed in paper machines is currently of an order of 25 meters per second. However, in the near future, it is anticipated that a speed range of from about 25 m/s to about 40 m/s is likely to be used. With the present highest running speeds and with the increasing running speeds in the future, in particular the dryer section has become, and will increasingly be, the bottle-neck in view of the runnability of a paper machine.
In addition, the requirements of quality imposed on the paper produced, in particular on fine paper and copying paper, are even now quite strict and are becoming ever stricter. Particularly high requirements are imposed on the symmetry of the paper in the z-direction and on the surface properties of both faces of the paper as well as on the stability of the structure of the paper as it is heated abruptly in a copying or printing process. These quality requirements imposed on a paper product impose particularly high requirements on the dryer section of a paper machine, and with increasing running speeds it is more and more difficult to meet these requirements.
In a manner known from the prior art, in multi-cylinder dryers of paper machines, twin-wire draw and/or single-wire draw is/are employed. In twin-wire draw, the groups of drying cylinders include two wires which press the web, one from above and the other one from below, against the heated cylinder faces. Between the rows of drying cylinders, which are usually horizontal rows, the web has free and unsupported draws, which are susceptible of fluttering resulting in web breaks. For this reason, in recent years, increasing use has been made of the single-wire draw in which each group of drying cylinders has only a single drying wire on whose support the web runs through the entire group so that the drying wire presses the web on the drying cylinders against the heated cylinder faces, whereas, on the reversing cylinders between the drying cylinders, the web remains at the side of the outside curve. Thus, in single-wire draw, the drying cylinders are situated outside the wire loop and the reversing cylinders are situated inside the wire loop.
In the prior art groups with single-wire draw, normal drying groups are used in which the heated drying cylinders are placed in an upper row and the reversing cylinders are placed in a lower row below the upper row of drying cylinder. The upper and lower rows are generally horizontal and parallel to one another. Further, as known from the prior art, besides the normal drying groups, dryer sections have also included so-called inverted groups with single-wire draw in which the heated drying cylinders are placed in the lower row and the reversing suction cylinders or rolls in the upper row, i.e., in an inverted formation. The principal objective of this arrangement is to dry the web symmetrically from both sides.
In dryer sections that comprise inverted and normal drying groups with single-wire draw, various problems have occurred, for which problems the present invention suggests novel and efficient solutions. These problems include the large length of the dryer section, which increases the costs of the dryer section and of the machine hall. Problems have also been encountered in the runnability of the dryer section and in the threading of the web. Problems also arise from differences in the speeds of different wires, as well as those problems related to the control of transverse shrinkage of the web. In inverted drying groups, in the event of breaks, a further problem consists of the difficulty in removing breaks because inverted groups are not self-cleaning by the force of gravity. Generally, these problems tend to become worse as the running speed of the paper machine becomes higher.
On the other hand, when dryer sections are used that include normal drying groups with single-wire draw alone, problems arise from the curling of the paper web, which results from the more intensive drying of the paper web from one side. For this reason, the inverted drying groups mentioned above have been used in dryer sections in spite of the problems described above. In the prior art arrangements, inverted groups have not been used in the initial part of the dryer section, because of the risk of web breaks, i.e., the dry solids content of the web at the initial part is not high enough to enable the Web to be passed in open draws existing in a twin-wire draw, but are primarily used toward the end of the dryer section. Frequently, besides the normal groups, just one inverted drying group has been employed.
In prior art dryer sections in which exclusively groups with single-wire draw are used, in the last wire groups, considerable wear of the drying fabrics has occurred, in particular in the manufacture of fine papers with a high content of fillers, and even more so if calcium carbonate has been used as a filler. Thus, the present invention is directed to reducing this problem which occurs in the drying groups driven by the drying wire because of the considerable thickness of the drying wire. The differences in speed mentioned above, together with a restricted drying shrinkage, cause web breaks in the last groups when exclusively single-wire draw has been used. This problem is emphasized further if, in the groups with single-wire draw, small-diameter suction rolls proper are used that are provided with an inside suction box. In order to eliminate this problem, in some machines, it has been even necessary to open some group gaps and to lower the level of negative pressure in the suction rolls. Problems similar or corresponding to those described above are encountered, e.g., in the dryer section described in the U.S. Pat. No. 5,269,074 (assigned to Beloit Corp.), wherein, in the groups with single-wire draw, normal suction rolls are used which have very small diameters and which are provided with inside suction boxes. Moreover, in this U.S. patent, a dryer section is described in which the last group is a single group with twin-wire draw, in which there are two rows of drying cylinders placed one row above the other, whereby the web has free draws between the two rows. A dryer section similar to that mentioned above is also described in the assignee's Finnish Patent Application No. 934367 (corresponding to the assignee's U.S. patent application Ser. No. 08/213,148, the specification of which is hereby incorporated by reference herein).
In the above-mentioned U.S. patent and Finnish patent application, it is indicated that the tensile strength of a paper web is increased substantially when its dry solids content becomes higher, so that it has been considered that the group with twin-wire draw should be placed preferably expressly as the last group in the dryer section in whose area the dry solids content of the web is at the maximum, and so is the tensile strength at the maximum in view of elimination of the problems of runnability produced by the free draws of the web. Then, consideration has, however, not been given to the fact that the susceptibility of breaks at the free draws of the web does not depend on the tensile strength of the web alone, but also on the breaking strength (tensile-energy absorption) and the tear strength in particular in respect of breaks and wrinkles starting in the lateral areas of the web. For this field of problem, a solution has been suggested in the assignee's Finnish Patent Application No. 940749 (filed on Feb. 17, 1994). In this Finnish patent application, among other things, a dryer section is described in which the second-to-last or penultimate group is a single group with twin-wire draw, in which there are two rows of drying cylinders, one row placed above the other, between which rows the web has free draws.